Solenoids, electromagnets, and other such devices are frequently employed to lock-out or otherwise control a variety of mechanical systems. The brake transmission safety interlock (BTSI) system utilized in motor vehicles is an example of a typical electromechanically controlled system. A BTSI system operates to prevent a vehicle's transmission from being taken out of the park position if the braking system is not activated. A typical BTSI system includes a solenoid, electromagnet or other such electromechanical device disposed in electrical communication with the brake light circuit of the vehicle and is operable to selectively engage and immobilize the shifter linkage if the brake is not activated.
Preload release is a significant concern in systems of this type. In a motor vehicle including a BTSI, preload release can occur when the operator attempts to shift the vehicle out of park prior to engaging the brake. The locking portion of the BTSI system is thereby placed under load and if this load is maintained, the locking system will fail to release even though the brake is subsequently activated. Only upon release of mechanical tension will the interlock release and allow proper shifting. Although preload binding would appear to be a minor problem since it can be simply and obviously overcome by releasing tension on the shift lever and since it can be prevented from occurring in the first place by using the shift and brake controls in proper sequence, driving habits are often very ingrained and automatic. It has been found that many complaints to automobile dealers and manufacturers regarding faulty transmissions are really attributable to operator induced preload binding in BTSI systems. As used within the context of this disclosure "Preload Release" refers to the ability of an interlock to properly release even when it is under a preloaded condition.
The second significant problem occasioned by BTSI systems occurs when there is a failure of the system which results in the permanent lock-up of the shifter linkage. Such failure can result from damage to the solenoid or other electromagnetic device or from failure of an electrical component energizing that device. In either event, the net effect is that the shifting linkage of the vehicle is locked and the driver cannot proceed. Clearly, such a failure can be a nuisance at best and an extreme hazard at worst. For this reason, automotive manufacturers generally prefer to have a "breakthrough" factor built into BTSI systems. By "breakthrough" is meant that the interlock feature of the system may be defeated by application of a significant amount of force to the shifter linkage. It has been found that a breakthrough force of approximately 45 to 75 lbs. is desirable since this force can be supplied by most vehicle operators if they make a conscious effort to do so; whereas, it is unlikely that such force would be inadvertently applied to the shifter linkage through accident or inadvertence. It should be noted that in some instances, breakthrough may not be required.
It will thus be appreciated that there is a need for a BTSI system which has preload capability and which can, in those applications where it is desired, be overcome by a breakthrough force in the approximate range of 45 to 75 lbs. It is also desirable that any such system be durable and readily adaptable to a variety of motor vehicle configurations. While the prior art has extensively addressed the field of BTSI systems, it has to date failed to provide any brake transmission safety interlock which can meet the aforementioned criteria.
U.S. Pat. No. 4,987,968 discloses a cable mounted BTSI system in which a solenoid causes a pair of balls to engage a notch in a rod associated with a shifter linkage. U.S. Pat. No. 5,018,610 discloses another interlock system in which a solenoid projects a plunger into a portion of a shifter or linkage. Preload binding has been found to be a significant problem with both of the foregoing types of systems.
As will be described in greater detail hereinbelow, the present invention provides an electromagnetic interlock which is not prone to preload binding. The interlock is readily adaptable for use in a BTSI system and provides for breakthrough at a readily selectable, and repeatable, level of force. The interlock of the present invention is simple and durable and exhibits a long service life. Its compact size and mechanical simplicity allow it to be readily adapted to a variety of applications. These and other features of the invention will be readily apparent from the drawings, discussion and description which follow.